1. Field of the Invention
This invention relates to the recorded video art and, more specifically, to a system for detecting and compensating for the loss of signals in the video recovered from the recording medium. The absence of either positive or negative going half cycles from the carrier signal is detected. Loss of individual half cycles result in the generation of a new half cycle of the appropriate polarity and its insertion in the carrier. If several consecutive half cycles are lost, informational content of the signal is retained and is applied to the system until the carrier is restored.
2. Description of the Prior Art
In the prior art, the recording and playback of the high-frequency, FM encoded signals which transmit video information and which may be of the order of 7.0 MHz, required the creation of special magnetic recording techniques and special problems developed. Different proprietary systems have been evolved, such as increasing the linear speed of the tape relative to a stationary head or using a plurality of rotating heads that rotate at relatively high speed at substantially right angles to a tape moving linearly. These systems have been used successfully to record and play back black and white as well as color television transmissions. However, whenever less than 100 percent of the original signal is recorded or reproduced for transmission to the display, the quality of the picture suffers. This is especially so if color television is involved.
The eye is extremely sensitive to drastic changes in the visual impression made to the eye no matter how fleeting. For example, a missing half cycle in an FM recorded carrier would have the effect of drastically reducing the frequency of the modulated signal at that instant. In many video recording systems, a decrease in the frequency represents a brightening of the picture. A missing half cycle has the effect of drastically lowering the frequency and is responded to by the television display system as an immediate and sudden, discontinuous lightening of the picture, sometimes called a "blooming" effect, which produces an intense white spot in what could be an otherwise "dark" region of a picture.
Such an instantaneous change in the brightness of a portion of the television display system is very disconcerting to the viewer and may be likened to static or "pops" and "clicks" emanating from a high fidelity sound recording. The white spot on the television screen is caused by the intensity amplifier being overdriven into a saturation range. The persistence of the screen maintains the spot for a relatively long time and the effect is compounded by the persistance of the image in the eye, disproportionately exaggerating the effect.
If several consecutive half cycles of carrier are lost, either due to imperfections in the recording medium or from malfunctions in the recording or playback system, portions of the video picture may be lost and "streaks" will appear in the display. This, too, is disconcerting to the viewer and represents a problem that must be solved to provide a commercially acceptable video recording and reproducing system suitable for the consumer.